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How does a Wavefunction collapse?The density matrix is related but it's not exactly my territory. I would refer you to this interesting discussion where the people who understand density matrices show that you cannot distinguish between a system where 50% of the atoms are in an excited state versus a system where all the atoms are in a 50-50 superposition of states: physics.stackexchange.com/questions/8049/…

How far back can you trace a photon?Then your argument is with Richard Feynmann, not me. I more or less quoted what he calls "Proposition A" (Vol. 3, chapter 1-5 in the Feynmann Lectures)...the very proposition which he concludes is false. According to Feynmann, it is simply not true that the photon must have passed through either one slit or the other.

Dec7

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How far back can you trace a photon?You start by saying you are going to "keep it simple" but then you throw in so many extraneous complications that your answer is all but incomprehensible. Let me list the things you bring up that have nothing to do with the question: the motion of the star relative to the earth, the expansion of space, existence of gravitational "wells" (?), the redshift, the blueshift, and the standard model. And why do you have a second potassium atom...and why do you think there are potassium atoms inside the detector? And how do you "see" an absorption line with a photomultiplier?

Dec7

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How far back can you trace a photon?If there is no energy spread out, then the energy must be concentrated. And in your "standard quantum mechanics", we have local conservation of energy, which means the energy must be somewhere. So you're saying the photon existed from the moment it left the potassium atom. I suppose you're saying if it encountered a double slit along the way, it must have passed through either one or the other of the slits.

Dec7

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How far back can you trace a photon?It seems like I'm saying it didn't become a photon until it entered the detector, and you're saying it was a photon from the moment it left the potassium atom.